J. Phys. Chem. 1989, 93, 2129-2133
Our spectral observations suggest that each is equally likely. In fact, observation of H-CEC-H from I , 1-dichloroethylene is a good illustration of the principle that all possible products are produced, as found for the deuteriated ethylenes. Electron-Impact Process. The main body of experimental evidence provided in this work indicates that electron bombardment causes primarily fragmentation and rearrangement reactions. These reactions must take place on the matrix surface during the condensation process where the gas density is high, since irradiating the sample by electrons after the matrix was formed gave no appreciable change in the spectra observed. Furthermore, when electrons were directed across the gas stream away from the matrix, no products were observed. In the production of fragments and rearrangement products, the matrix must play an important role either (i) by providing a higher molecular density or (ii) by providing the third body to dissipate the extra energy such that rearrangement reactions can occur. Certainly, the matrix is essential for trapping the fragments together in the same cage after the initial rupture of the bonds. All of this evidence suggests that electron impact is most effective just before the fragments are frozen into the matrix and not in the gas phase nor after the matrix is formed. In this respect it is appropriate to compare the matrix electron-impact results with high-pressure mass spectrometry results and to state that the majority of products formed by electron impact in a high-density gas are the neutral species and that ions are produced in much smaller concentrations. Conclusions In this work, we have described the use of I R absorption spectroscopy to study the well-known electron-impact process on dilute argon/hydrocarbon gaseous mixtures during their con-
2129
densation onto a CsI window at 12 K. The electron-impact process takes place on the sample surface where the condensing gas density is high. Surprisingly the major products were neutral species; although some ion formation cannot be ruled out, no significant spectroscopic evidence was found for trapped ions.21 Diatomic molecular elimination seems to be the most favorable fragmentation channel. Both 1,l- and 1,2-elimination were observed from isotopic and substituted ethylenes. Fragments trapped together in the matrix cage formed complexes as evidenced by perturbed IR absorptions; in this respect electron impact is very similar to vacuum-UV photolysis. In addition, rearrangements leading to various isomers were observed by electron impact including 1,lC2HZD2to cis- and trans-CHDCHC and a 1,3-sigmatropichydride shift in acrylonitrile to give the H2C==C=C=N-H species. The most important conclusion from this matrix isolation study of electron impact on hydrocarbons is that netural molecules and fragments were the major products, and charged fragments were not observed here. Acknowledgment. We gratefully acknowledge financial support from the Thomas F. and Kate Miller Jeffress Memorial Trust and the communication of results on vinyl radical by W. R. M. Graham before publication. Registry NO. C&74-84-0; C2D6, 1632-99-1;CzH4, 74-85-1; CH2CD2, 6755-54-0; CzD4,683-73-8; C2H3C1,75-01-4; C2H3Br,593-60-2; C2H3CN,107-13-1; 1,1-C2H2Cl2,7 5 - 3 4 ; cis-C2H2CI2,156-59-2; c6HSCI, 108-90-7. (21) A weak product bandt0 at 697 cm-I in vinyl chloride experiments could be due to HCI,, but its position on the shoulder of a strong vinyl chloride band at 712 cm-' requires caution for this identification.
Rigidochromism as a Probe of Gelation and Densification of Silicon and Mixed Aluminum-Silicon Aikoxides John McKiernan,t Jean-Claude Pouxvie1,t Bruce Dunn,tq* and Jeffrey I. Zinkt** Department of Chemistry and Biochemistry and the Department of Materials Science and Engineering, University of California, Los Angeles. Los Angeles. California 90024 (Received: June 7, 1988)
The emission maximum of ReC1(CO)3bpy blue shifts as a function of increasing rigidity of the surrounding matrix. This rigidochromic effect provides a sensitive means of probing the structural changes that occur in the sol-gel process. Two sol-gel systems, tetraethoxysilane (TEOS) and a mixed aluminosilicate system (ASE), are studied. The magnitudes of the shifts of the luminescence maximum of the rigidochromic probe molecule were established in relevant test systems by using fluid and frozen solid ethanol and TEOS as test matrices. Shifts of about 2500 cm-' were found. The changes in the luminescence were then followed during the sol-gel-solid transformation of the aluminosilicate and TEOS systems. In the former, the emission maximum monitors the initial partial rigidification during aging and further follows the subsequent rigidification during drying. In the TEOS system, the emission of the probe does not shift during aging but exhibits a large change during drying. These contrasting results show that on the molecular level the two systems have quite different structural properties. The aluminosilicate gel contains small pores that trap the dye molecules and large pores that enable the solvent to diffuse. The small pores partially contract during aging without macroscopic changes of the gel. Further shrinkage occurs during drying. In contrast, in the TEOS system the ReC1(C0)3bpy molecules are not encapsulated into the gel and instead remain in the interstitial liquid phase. The molecules adsorb on the silica surface of the pore walls at the last stage of the drying.
The sol-gel process is a synthesis technique recently developed to prepare gels, glasses, and ceramic powders.' Metal alkoxides (formula M(OR),, where M is Al, Si, Ti, . . ., and R is an organic group) undergo hydrolysis (eq 1) followed by polycondensation reactions in solution at room temperature (eq 2). When the rates M(0R) H20 M(0H) ROH (1) M(0H) + M(0H) M-0-M + H20 (2)
+
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Department of Chemistry. *Department of Materials Science.
0022-3654/89/2093-2129$01.50/0
of the reactions are well controlled, the solution becomes increasingly more viscous as an inorganic polymer M-0-M grows. Eventually the solution turns into a transparent amorphous solid. At this stage, the material is a rigid gel that still contains water and organic solvents. Subsequent drying at room temperature or 70 'C removes most of the organic molecules. The resulting (1) For general references on the sol-gel process, see Glass and Glass Ceramics From Gels. J . Non-Cryst. Solids 1986, 82, and Better Ceramics Through Chemistry II; Brinker, C . J., Clark, D. E., Ulrich, D. R., Eds.; MRS Symposium, Vol. 73.
0 1989 American Chemical Society
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material is an amorphous, transparent solid that contains pores of varying sizes (700 "C) converts these gels into fully dense glasses. The initial low processing temperature makes it possible to dope these gels with organic and organometallic molecules that would not remain intact at the temperatures normally used for molten silicate glass. To incorporate organics, they are dissolved either in water or in the same alcoholic solvent used for the alkoxide precursor. Avnir et al. have reported the preparation and the optical properties of alkoxide-derived silica gels doped with rhodamine 6G and p~rene.~' They have also demonstrated that other organic molecules can be successfully incorporated into sol-gel mat rice^.^,^ The number of potential applications of such systems is very large; important possibilities include nonlinear optical devices, luminescent solar concentrators, and chemical sensors. Development of these applications requires a good understanding of the structure of the doped sol-gel matrices, the properties of the matrices on the molecular level, and the conditions that the oxide network imposes on the optical properties of the dopant. Little is known thus far concerning the fundamental aspects of the interaction between organic dyes and the inorganic host matrix when molecules are truly entrapped and not merely adsorbed on the surface of the exterior or the pores. Potentially useful probes for monitoring the interactions between the guest dopants and the host matrix during the transformation from the sol to the dried glass are the "rigidochromic" molecules ReCl(CO),L (L = bidentate d i i m i ~ ~ e ) .The ~ , ~luminescence of these molecules changes markedly with changes in the rigidity of the medium. Specifically, the wavelength of the emission shifts to the blue (higher energy) and the quantum yield increases in rigid media compared to the values of these properties in fluid solutions. Changes in the energy of the order of 2000 cm-I have been ob~ewed.69~ If the internal changes occurring during gelation are large enough to produce measurable shifts in the emission energy of a rigidochromic molecule incorporated in the system, the luminescence spectra could provide a direct measure of the rigidity of the system on the molecular level. We report here the use of the rigidochromic molecule bipyridyltriscarbonylchlororhenium(I), ReCI(CO),bpy, as a probe of the changes in rigidity of sols and gels prepared from tetraethoxysilane (TEOS) and (diisobutoxya1umino)triethoxysilane (OBu),Al-@Si(OEt), (or ASE). The changes in the energy of the luminescence maximum are measured in fluid and solid (frozen) reference solvents to calibrate the magnitude of the rigidochromic effect. The changes are then used to probe the rigidity of the two systems during gelation and drying. These studies reveal large differences between the two systems.
Experimental Section The rigidochromic molecule ReCl(CO),bpy was synthesized by using the literature method.6 Two kinds of matrices were prepared and doped with ReCl(CO),bpy to probe rigidity changes during gelation and densification of silica sonogels and aluminosilicate gels. The silica sonogels were made by mixing 15 mL of TEOS and 5 mL of distilled water in an ultrasonic bath and then adding 3 drops of concentrated aqueous HCl to catalyze the reaction. After the reaction had proceeded for 5 min, 5 mL of a M solution of ReCl(C0)3bpy in 95% ethanol was added together with 15 mL of TEOS and 5 mL of water. The solution was then placed in the ultrasonic bath for an additional 10 min. The ASE gels were prepared by following the literature procedure
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.
. 21
.
. 19
.
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.
. 15
.
. 13
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for the hydrolysis of the mixed alkoxide (diisobutoxya1umino)triethoxysilane (OBu),Al-O-Si(OEt), supplied by Dynamit Nobel."lo Gels having good transparency, convenient mechanical resistance, and short gelation times were obtained by mixing and stirring a solution composed of 10 mL of 2-propanol and 10 mL of ASE with a mixture of 5 mL of water and 10 mL of propanol. The dopant concentration was also 10" mol/L. Both types of sols were contained in closed styrene cuvettes. Gelation occurred within a few days at room temperature. The cuvettes were kept closed for 10 more days to allow aging of the gels without any loss of solvent. Finally, a small hole was pierced in the cover to allow slow evaporation of alcohol and water. Front face fluorescence spectra were recorded on a Spex Fluorolog spectrophotometer, Model No. F112A, with an excitation wavelength of 350 nm. Spectra from liquid samples and wet gels were measured from the styrene cuvettes, while dried gels were measured in air by using a special sample holder. Lowtemperature spectra were measured by exciting the sample in an Air Products displex with the 352-nm argon ion laser line and recording the spectra with a 3/4-m monochromator, a C31034 photomultiplier, and photon counting."
Results Luminescence Spectroscopy in Relevant Solvents. The luminescence spectra of ReCl(CO),bpy in ethanol and TEOS as a function of temperature were measured in order to verify the shift of the luminescence maximum with the rigidity of the medium and to determine the magnitude of such shifts. In these studies, the rigidity of the medium was changed by freezing the solution. For verification that the rigidity and not the temperature was responsible for the shifts, the spectra of the molecule were obtained as a function of temperature in a rigid gel. This gel was prepared as described above with the exception that the sol-gel process occurred in open cuvettes. The changes in the emission spectrum of ReCl(CO),bpy in ethanol as a function of temperature are shown in Figure 1. At room temperature the emission maximum is 6 12 nm ( 16 340 cm-I). At temperatures well below the freezing point of ethanol, the emission maximum is 529 nm (18 900 cm-'). A plot of the wavelength of the emission maximum as a function of temperature is shown in Figure 2. The error bars represent the uncertainty in measuring the position of the broad emission peak. Within the experimental uncertainty, the emission maximum remains unchanged in the temperature range between about 175 and 300
(2) Avnir, D.; Kaufman, R.; Reisfeld, R. J. Non-Crysr. Solids 1985, 74, 395.
(3) Kaufman, V.; Avnir, D. Langmuir 1986, 2, 717. (4) Avnir, D.; Levy, D.; Reisfeld, R. J. Phys. Chem. 1984, 88, 5956. (5) Kaufman, V. R.; Avnir, D.; Pines-Rojanski, D.; Huppert, D. J. NonCrysr. Solids 1988, 99, 379. (6) Wrighton, M. S.; Morse, D. L. J. Am. Chem. SOC.1974, 96, 998. (7) Giordano, P. J.; Wrighton, M. S . J. Am. Chem. Soc. 1979, 101, 2888.
(8) Pouxviel, J. C.; Boilot, J. P. J. Muter. Science, in press. (9) Pouxviel, J. C.; Boilot, J. P.; Lecomte, A.; Dauger, A. J. Phys. (Puris) 1987, 48, 921. (10) Boilot, J. P.; Pouxviel, J. C.; Dauger, A,; Wright, A. In Berrer Ce-
ramics Through Chemistry III; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; MRS Symposium, Reno, NV, April 1988. ( 1 1 ) Tutt, L.; Zink, J. I. J. Am. Chem. SOC.1986, 108, 5830.
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2131
Rigidochromism as a Probe of Two Sol-Gel Systems
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480
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Temperature (K) Figure 2. Plot of the wavelength of the emission maxima (nm) of ReC1(CO)3bpy in ethanol as a function of temperature (K). =" I
Figure 4. Emission spectra of ReC1(C0)3bpy in the aluminosilicate system at room temperature: (1) initial liquid, (2) aged gel, (3) dried
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Temperature (K) Figure 3. Plot of the wavelength of the emission maxima (nm) of ReC1(CO)3bpy as a function of temperature in TEOS sol (top) and in
the rigid gel (bottom).
K. As the freezing point of ethanol is approached at temperatures below 175 K, the emission maximum blue-shifts. At the freezing point of the mixture, the emission maximum shows a sharp decrease in wavelength. After freezing, the emission maximum shows a further small blue shift as the temperature is decreased to about 100 K, after which no further shift is observed. The behavior of the emission maximum as a function of temperature in liquid TEOS before gelation is shown in Figure 3. This behavior is similar to that observed in ethanol. Above the freezing point of the solution, the emission maximum is 605 nm (16 530 cm-I). As the temperature is decreased, the emission maximum is virtually unchanged down to a temperature of about 180 K. A sudden blue shift is observed in the region of the freezing point of the mixture. The band maximum in the frozen solution is about 530 nm (18 870 cm-'). The emission spectra of ReC1(CO)3bpy in the dried gel formed from TEOS were studied for comparison with the fluid solution/frozen solution data. The position of the band maximum as a function of temperature in this medium is shown in Figure 3. The position of the band maximum is independent of temperature over the range studied. It coincides with the position of the band maximum in the solid frozen solution of TEOS within experimental error. Luminescence Spectroscopy in the Aluminosilicate Sol and Gel. The band maximum of the emission of ReC1(C0)3bpy in ASE exhibits large shifts at various stages of the gelation and drying processes. The emission band in the liquid state just after the mixing of the components is broad and centered around 600 nm (v = 16660 cm-'; A v l j 2 = 2500 cm-'). No change is observed at the gelation point, although the solution has turned to a solid amorphous material that does not flow when the container is tilted.
Figure 5. Plot of the wavelength of the emission maxima of ReCI(CO)3bpyin the aluminosilicate system as a function of processing time (lowest curve). The gelation point, the aging period, and the drying
period are shown. The concomitant weight changes are shown in the upper curve. After a long period of aging in a closed container, the maximum ~ emission is blue-shifted to 560 nm (v = 17 860 cm-I; A V ~=,3650 cm-'). At the end of the drying, the emission peak has shifted to 523 nm (v = 19 120 cm-l) and has a width of about 3580 cm-I. The small change in the fwhm indicates that the position of the band maximum has uniformly shifted and that the effect is not caused by the growth of a new band with a concomitant decline of the original band. The progression of changes in the emission band is shown in Figure 4. The shifts of the position of the band maximum and changes in the weight of the sample as a function of time are shown in Figure 5. For convenience, the processing time is plotted on a logarithmic scale. The evolution of ReC1(C0)3bpy emission as a function of time is composed of two steps. The first consists of a continuous shift of the luminescence maximum to 560 nm. No loss of solvent occurs during this time period. Surprisingly, the shift does not occur during the sol-gel transition but instead begins a t a later time during the aging of the gels. After a time span of about 5-6 times the gelation time, the shift of the emission maximum ceases. The second step is clearly related to the drying of the gel. The shift of the emission maximum occurs continuously with the decrease in weight of the sample due to removal of solvents (initially representing nearly 80% in volume of the gel). The linear shrinkage ( A l l l ) of the gel after drying is about 50% and causes an 8-fold increase in the concentration of ReC1(CO)3bpy. Luminescence Spectroscopy in TEOS Sol and Gel. Exactly the same procedure is followed for the preparation of silica sonogels containing the rigidochromic probe. However, the TEOS system doped with ReCl(CO),bpy exhibits a very different behavior. From the initial liquid state to almost the end of drying, the emission maximum remains fixed at about 600 nm. A blue shift only occurs with the final removal of the alcohol. In addition, luminescence bands from intrinsic defects in the silica network
2132 The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
are observed at 420 and 470 nm. This new emission is also found in undoped samples prepared by following the same procedure. The presence of these new bands makes it difficult to determine the exact position of the ReCl(CO),bpy luminescence maximum because of the overlap of the peaks. However, the results indicate that no blue emission shift occurs until the last stages of the drying. The emission of ReCI(CO)3bpy in dried silica sonogels can be observed if the solvents are allowed to evaporate during gelation (Figure 2). In this case, the intrinsic emission band of the gel is less intense, and this makes it possible to observe the probe fluorescence at 530 nm.
Discussion I . Rigidochromism. The large shift of the wavelength of the emission maximum of ReCl(CO),bpy with changes in the rigidity of the medium has been called “ r i g i d o c h r ~ m i s m ” .This ~ ~ ~ phenomenon has been obse,wed in the spectra of a variety of complexes of the form ReCl(C0)3L where L is bipyridine, 1,lOphenanthroline, and substituted phenanthrolines. In all of these compounds the emission has been assigned to a rhenium to diimine ligand charge-transfer excited state. The lifetimes are in the lo-’ to 10” second range, suggesting that the emission has considerable triplet to singlet c h a r a ~ t e r . ~The . ~ origin of the rigidochromic effect has not yet been e s t a b l i ~ h e d . ~Rigidochromic ~~J~ behavior is not observed for the compounds Re(CO)5X (X = C1, Br)I3 or ReCI( CO),( p h ~ s p h i n e )l ~4 . The data shown in Figure 3 offer proof that the shift of the wavelength of the emission maximum is caused by rigidity and not solely by changes in the temperature. When ReC1(C0)3bpy is dissolved in liquid TEOS and the emission maximum monitored as a function of temperature, a sudden blue shift occurs at the freezing point of the mixture. The suddenness of the shift suggests that it is the solidification of the mixture that causes the shift and not just the temperature change. If the change in temperature were the only governing factor, the shift would be expected to be monotonic with temperature. When ReC1(C0)3bpy is incorporated in TEOS sols and subsequently in gels, Le., in the rigid medium, no shift as a function of temperature is observed. This experiment proves that the spectroscopic shift is not caused by temperature. The medium is rigid throughout the temperature range studied, and the band maximum is unshifted. The wavelength of the band maximum in the rigid gel, 520 nm, is very close to the values observed in the rigid frozen TEOS and ethanol solutions ( 5 3 0 nm). The small difference is probably a result of the effects of different medium polarities on the position of the charge-transfer band maximum. Solvent-polarity-induced shifts of this magnitude are well-known for charge-transfer emission bands in organometallic compounds.12 The details of the shifts in the band maximum as a function of temperature in Figures 2 and 3 are also of interest. In the region of the freezing point of the medium, the shift is not completely discontinuous. The largest change occurs between the points in the figures labeled liquid and solid where the medium visually changes from fluid to solid. However, the onset of the shift occurs several tens of degrees above the temperature at which the mixture solidifies. The emission of ReCl(CO),bpy is probably responding to the increase in the viscosity as the freezing point is approached. Similar behavior is observed in a small temperature range below the freezing point of the mixture; Le., the blue shift continues for a few tens of degrees after the mixture becomes solid. These results indicate that the emission spectrum of ReC1(C0)3bpy is a sensitive probe of the degree of rigidity. Note that the emission band shown in Figure 1 is broad. This breadth prevents the exact position of the band maximum from being measured with an accuracy of better than about f10 nm. Consequently, it is not meaningful to attempt to overinterpret the fine details of the shift in the region of the freezing point of the medium. (12) Lees, A. J. Chem. Rev. 1987,87, 71 1. (13) Geoffroy, G. L.; Wrighton, M. Organometallic Photochemistry; Academic Press: New York, 1979. (14) Henein, M. N.; Zink, J. I., to be submitted for publication.
McKiernan et al.
2. ReCl(CO),bpy as a Probe of Gel Rigidity. The evolution of the ReCl(CO),bpy luminescence shown in Figure 5 can be separated into three distinct parts, each of them related to a specific sequential step of the sol-gel transformation: gelation, aging, and drying. First, during the gelation stage, the most drastic chemical and physical changes occur. The initial sol, which is completely fluid, is transformed into a solid and brittle material. After the addition of water, small particles on the order of 10-20 A in diameter are rapidly formed as a consequence of the hydrolysis of the Al-OR groups to Al-OH and the condensation of the AI-OH groups into -Al-O-AI- polymers. After this step, a slow aggregation process takes place, which forms inorganic clusters of increasing size. These clusters have been characterized and have a ramified and open s t r u c t ~ r e . ~ .When ’ ~ a critical cluster size is reached, the viscosity of the sols sharply increases and the solution becomes rigid.9,’0 Interestingly, the ReCI(CO),bpy probe molecule is insensitive to these changes, and the emission peak maximum remains constant at 590 nm. This observation indicates that although the solution is macroscopically solid, the rigidochromic molecules are unconstrained in a solvent-like environment. In contrast, the large changes caused by the solidification during the freezing of the solvents (vide supra) show that the macroscopic solidification is accompanied by rigidification of the matrix on the molecular level. Thus, in the case of the solidification caused by gelation, the spectroscopic results show that the macroscopic rigidification of the gel is not accompanied by rigidity at the microscopic level. The sofvent phase at the gel point consists of propanol, water (Le., the water not used for hydrolysis or released by condensation reactions), and the additional ethanol and butanol produced by the hydrolysis of the alkoxy groups. This phase constitutes the largest volume fraction of the gel. The solid-phase oxide network that causes the observed macroscopic rigidity only represents a small percentage of the total gel volume. At this stage, the mobility of the ReC1(C0)3bpy molecules is not constrained by the ramified and open gel structure. Subsequently, the luminescence is the same as in the liquid sol. The second step of the process, the aging period, takes place in a sealed vessel. Although the gel is kept in a closed container and no evaporation of the organic molecules occurs, the luminescence maximum continuously blue-shifts. At the end of the aging period, the emission maximum stabilizes at 560 nm, intermediate between the value observed in a completely fluid medium, 600 nm, and that in a completely solid medium, 525 nm. The blue shift during this period is caused by partial rigidification of the gel on the molecular level. It indicates that the ReCl(C0)3bpy probe molecules are sensitive to structural changes in the aluminosilicate gel. The ,rigidochromic probe is not simply surrounded by the interstitial liquid phase as it was during the gelation stage, but now it becomes somewhat entrapped in the oxide polymer network. At the gel point, the gel network is a very ramified and open structure that does not restrict the mobility of the rigidochromic molecules. The evolution of the fluorescence during the aging period shows that the motion of ReCl(CO),bpy molecules is progressively restricted. These results indicate that large modifications in the local gel structure occur during the aging period, which increase the rigidity of the oxide skeleton. These structural modifications are in agreement with the reported chemical changes in the surface of the oxide network of the aluminosilicate probed by the luminescence of pyranine.’’ Condensation reactions between A1-OH groups are probably responsible for the local evolution of the polymers, which leads to a more compact network and increases the gel rigidity during the aging process. The structure of the polymer network is controlled by the diffusion of clusters during the aggregation stage and by the rates of the chemical reactions that build the network. The resulting structure is not necessarily the most thermodynamically stable state. The clusters tend to form a very open and ramified structure (also described as mass fractal ~ b j e c t ) . ~Thus, J~ the structure is characterized by a large internal surface that gives (15) Pouxviel, J. C.; D u m ,
B.;Zink, J. J . Phys. Chem., in press.
Rigidochromism as a Probe of Two Sol-Gel Systems rise to an appreciable surface energy. There is still some flexibility in the network. Molecular transport of reactants and products is possible through the interstitial liquid phase, and continued hydrolysis and condensation reactions are, thetefore, highly probable. These reactions and thermodynamic considerations mean that during aging, the gel evolves toward a more compact state with a smaller internal surface. The observation that the gel keeps the same macroscopic size and shape indicates that these structural changes are occurring at virtually the atomic level (a few tens of angstroms). In order to be sensitive to these changes the ReC1(C0)3bpy molecules must be trapped within the part of the gels that undergoes this evolution. The local strains induced in the gel are probably strong enough to provide a partially rigid environment for the ReC1(C0)3bpy molecules. One macroscopically observable property that is indicative of these changes is the gel hardness, which increases by a factor of 5 during the same period of time.I6 The third stage of the process, which is related to the final changes of the emission of ReC1(CO)3bpy, is the drying of the gel. When the solvents are removed, the gel structure collapses and the gel shrinks continuously. The final volume is about one-eighth of that of the aged gel. As indicated by the blue shift of the emission of ReC1(CO)3bpy, this step is accompanied by a progressive and eventually complete rigidification of the matrix. The final emission wavelength is the same as that in frozen ethanol solution. Both the increase in surface energy due to modification of the interface between the oxide polymer and the liquid and the capillary forces of the solvents provideathedriving force for this partial densification. New condensation reacfions occur because some reactive groups are in closer proximity. As a result, the oxide network becomes more compact. Its flexibility drastically decreases with the departure of the solvating species. The ReCl(C0)3bpy molecules, which were previously entrapped in the network, are now completely immobilized by this collapsing structure. Moreover, the probe molecules cannot be leached from the gel. It is usually during the drying process that gels develop cracks because of the increasing rigidity and the capillaryforce-induced strains that cannot be released by changes in the gel shape and morphology. 3. Behavior of TEOS Gels. The rigidochromic probe molecule was also used to study the gelation, aging, and drying of the TEOS system. The probe showed quite different behavior compared to that in the aluminosilicate system. The wavelength of the emission maximum remains constant at 590 nm from the initial liquid state through the gelation process to almost the end of the drying. The blue shift indicative of rigidity occurred only with the final removal of the organic molecules. This contrasting behavior of the silica sonogel indicates that there are significant structural differences between the two kinds of investigated systems and that ReC1(CO)3bpy is a sensitive probe of this difference. In the silica sonogels, the constancy of the emission at 590 nm indicates that the probe molecules are in a nonrigid environment during gelation, aging, and a major period of the drying. The fact that this (16) Pouxviel, J. C. Ph.D. Thesis, University of Paris VI, 1987.
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2133 behavior is different from that of aluminosilicate gels is not surprising. Indeed, the chemistry of the precursors, the mechanisms of polymerization, and therefore the structure of the gels are different.” Moreover, the characteristics of the pore walls, including polarity and electric charge, are expected to be substantially different. This may change the interactions of the dye with the oxide network and consequently its psition inside the gel structure. In the TEOS-derived gel, the blue shift only occurs with the final loss of solvents, suggesting that during gelation and aging the probe molecules are confined in the interstitial liquid phase and are not incorporated into the silica polymer network. The adsorption of the active dyes on the surface of a pore once the water and ethanol are almost completely evaporated may account for the evolution of the ReC1(C0)3bpy fluorescence toward the characteristic blue shift of the rigidified state. In the TEOS system, it is impossible to exactly follow the final shift of the maximum on the emission spectrum of ReC1(C0)3bpy incorporated into the rigid system because of the overlap of the luminescence peak with an intense peak from the intrinsic emission of the gel. It is clear, however, that the peak has shifted to about 530 nm, indicative of a rigid system. Summary. The emission maximum of ReC1(C0)3bpy blueshifts as a function of increasing rigidity of the surrounding matrix. This rigidochromic effect provides a sensitive means of probing the structural changes during gelation, aging, and drying of silica and aluminosilicate get systems. The magnitude of the shifts in the luminescence maximum of the probe molecule were edablished by using fluid and frozen solid ethanol and TEOS as test matrices. The changes in the luminescence were then followed during the sol-gel-solid transformation of the aluminosilicate and TEOS systems. In the former, the emission maximum monitors the initial partial rigidification during aging and further follows the subsequent rigidification during drying. In the TEOS system, the emission of the probe does not shift during aging but exhibits a large change during drying. The results show that the two systems have significantly different behavior with respect to the luminescence changes of ReC1(C0)3bpy. In the case of the aluminosilicate gel, the probe molecules are entrapped within the fine structure of the polymer network. This small pore system collapses during aging, involving a partial rigidification of the ReC1(CO)3bpy molecules. During the drying stage, the whole structure of the gel shrinks continuously, progressively causing the complete rigidification of environment for the probe molecules. In contrast, the probe molecules remain in the interstitial liquid phase of the TEOS-derived silica gel until the final removal of the solvents. The rigidification of the ReC1(C0)3bpy only occurs with the adsorption of these molecules onto the gel surface. Acknowledgment. This work was made possible by a grant from the National Science Foundation (NSF DMR 87-06010). Registry No. TEOS, 78-10-4; EtOH, 64-17-5; ReC1(CO)3bpy, 55658-96-3. (17) Kaufman, V. R.; Muller, S.C.; Muller, K. H.; Avnir, D. G e m . Phys. Lett. 1987, 142, 551.
